AB 1756-EN2T 通讯模块电流输出型变送器将物理量转换成4 – 20mA电流输出， 必然要有外电源为其供电。最典型的是变送器需要两根电源线， 加上两根电流输出线，总共要接4根线， 称之为四线制变送器。电流输出可以与电源公用一根线（公用VCC或者GND ）， 可节省一根线， 称之为三线制变送器。4- 20mA电流本身就可以为变送器供电， 见图2.变送器在电路中相当于一个特殊的负载，特殊之处在于变送器的耗电电流在4- 20mA 之间是根据传感器输出而变化的。这种变送器只需外接2根线， 因而被称为两线制变送器。
1756-EN2T
工业电流环标准下限为4mA, 因此只要在量程范围内， 变送器至少有4mA 供电。AB 1756-EN2T 通讯模块这使得两线制变送器的设计和使用成为可能。而且两线制变送器具有无可比拟的优点：不易受寄生热电偶和沿电线电阻压降和温漂的影响， 可用非常便宜的较细的导线； 可节省大量电缆线和安装费用。在电流源输出电阻足够大时， 经磁场耦合感应到导线环路内的电压， 不会产生显著影响，因为干扰源引起的电流极小， 一般利用双绞线就能降低干扰； 两线制与三线制必须用屏蔽线， 屏蔽线的屏蔽层要妥善接地。
1756-EN2T
AB 1756-EN2T 通讯模块电容性干扰会导致接收器电阻产生有关误差， 对于4~ 20mA 两线制环路， 接收器电阻通常为2508 （取样Uou t= 1~ 5V ） 这个电阻小到不足以产生显著误差，因此，可以允许的电线长度比电压遥测系统更长更远。各个单台示读装置或记录装置可以在电线长度不等的不同通道间进行换接， 不因电线长度的不等而造成精度的差异， 从而实现分散采集、集中控制。将4mA 用于零电平， 使判断开路与短路或传感器损坏（ 0mA 状态） 十分方便。在两线输出口非常容易增设一两只防雷防浪涌器件， 有利于安全防雷防爆。
1756-EN2T
The AB 1756-EN2T communication module current output transmitter converts physical quantities into 4-20mA current output, and external power supply is necessary for its power supply. The most typical is that the transmitter requires two power lines, plus two current output lines, to connect a total of four lines, which is called a four wire transmitter. The current output can be shared with the power supply on the same wire (common VCC or GND), which can save one wire and is called a three wire transmitter. The 4-20mA current itself can supply power to the transmitter, as shown in Figure 2. The transmitter is equivalent to a special load in the circuit, and the special feature is that the power consumption current of the transmitter varies between 4-20mA based on the sensor output. This type of transmitter only requires 2 external wires, hence it is called a two wire transmitter.
1756-EN2T
The standard lower limit of the industrial current loop is 4mA, so as long as it is within the range, the transmitter should have at least 4mA power supply. The AB 1756-EN2T communication module makes it possible to design and use a two wire transmitter. Moreover, the two wire system transmitter has unparalleled advantages: it is not easily affected by parasitic thermocouples and resistance drop and temperature drift along the wire, and can be used with very cheap and thin wires; It can save a lot of cable and installation costs. When the output resistance of the current source is large enough, the voltage inside the wire loop induced by magnetic field coupling will not have a significant impact, because the current caused by the interference source is very small, and the interference can generally be reduced by using twisted pairs; Two wire and three wire systems must use shielded wire, and the shielding layer of the shielded wire must be properly grounded.
1756-EN2T
The capacitive interference of the AB 1756-EN2T communication module can cause errors in the receiver resistance. For a 4-20mA two wire loop, the receiver resistance is usually 2508 (sampling Uou t=1-5V), which is too small to produce significant errors. Therefore, the allowable wire length is longer and farther than that of the voltage telemetry system. Each single reading device or recording device can be swapped between different channels with different wire lengths, without causing accuracy differences due to wire lengths, thus achieving decentralized collection and centralized control. Using 4mA for zero level makes it very convenient to determine open circuit, short circuit, or sensor damage (0mA state). It is very easy to add one or two lightning and surge protection devices at the two line output ports, which is conducive to safe lightning protection and explosion protection.
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